feat(scep): mTLS sibling route /scep-mtls/<pathID> (opt-in)

SCEP RFC 8894 + Intune master bundle — Phase 6.5 of 14 (opt-in, enterprise-procurement-checkbox). Closes the procurement-team objection that 'shared password authentication' is a checkbox-fail regardless of how strong the password is. The clean answer: a sibling route that adds client-cert auth at the handler layer AND keeps the challenge password (defense in depth, not replacement). Devices present a bootstrap cert from a trusted CA (e.g. a manufacturing-time cert), then SCEP-enroll for their long-lived cert. Same model Apple's MDM and Cisco's BRSKI use. internal/config/config.go * SCEPProfileConfig gains MTLSEnabled bool + MTLSClientCATrustBundlePath string. Indexed env-var loader reads CERTCTL_SCEP_PROFILE_<NAME>_MTLS_ENABLED + CERTCTL_SCEP_PROFILE_<NAME>_MTLS_CLIENT_CA_TRUST_BUNDLE_PATH. * Validate() refuses MTLSEnabled=true with empty bundle path — structural defense in depth ahead of the file-content preflight. cmd/server/main.go * preflightSCEPMTLSTrustBundle: file existence + PEM parse + ≥1 CERTIFICATE block + non-expired check. Returns the parsed *x509.CertPool ready to inject into the per-profile SCEPHandler. Failures os.Exit(1) with the offending PathID in the structured log. * SCEP startup loop walks each profile; when MTLSEnabled, runs preflight, builds the per-profile pool, contributes the bundle's certs to the union pool that backs the TLS-layer VerifyClientCertIfGiven, clones the SCEPHandler with SetMTLSTrustPool, and registers the parallel sibling route via apiRouter.RegisterSCEPMTLSHandlers. * Union pool published to outer scope as scepMTLSUnionPoolForTLS; passed to buildServerTLSConfigWithMTLS so the listener serves both /scep[/<pathID>] (no client cert) and /scep-mtls/<pathID> (cert required at handler layer) on the same socket. * Final-handler dispatch gains /scep-mtls + /scep-mtls/* prefix routing through the no-auth chain (auth boundary is the client cert + challenge password, NOT a Bearer token). cmd/server/tls.go * New buildServerTLSConfigWithMTLS that wraps buildServerTLSConfig + sets ClientCAs + ClientAuth=VerifyClientCertIfGiven when a non-nil pool is passed. nil pool = identical TLS shape to the pre-Phase-6.5 builder (no behavior change for deploys without mTLS profiles). * Critical: VerifyClientCertIfGiven (NOT RequireAndVerifyClientCert) so a client that doesn't present a cert can still hit the standard /scep route. The per-profile gate at the handler layer enforces 'cert required' on /scep-mtls/<pathID>. internal/api/handler/scep.go * SCEPHandler gains mtlsTrustPool *x509.CertPool field + SetMTLSTrustPool method. Per-profile pool injected by cmd/server/main.go after preflight. * HandleSCEPMTLS wrapper: gates on r.TLS.PeerCertificates non-empty + per-profile cert.Verify against THIS profile's pool. Returns HTTP 401 for missing/untrusted cert (mTLS failure is auth, not authorization). Returns HTTP 500 if mtlsTrustPool is nil (deploy bug — the route shouldn't have been registered). On success delegates to HandleSCEP — defense in depth: mTLS is additive, NOT replacement; the standard SCEP code path including the challenge-password gate still executes. * Per-profile re-verification via cert.Verify(...) is critical: the TLS layer verified against the UNION pool, so a cert that chains to profile A's bundle would pass TLS even when targeting profile B. The handler-layer gate prevents cross-profile bleed-through. internal/api/router/router.go * AuthExemptDispatchPrefixes gains '/scep-mtls' (auth boundary is client cert + challenge password, NOT Bearer token). * RegisterSCEPMTLSHandlers parallel to RegisterSCEPHandlers: empty PathID maps to /scep-mtls root; non-empty maps to /scep-mtls/<pathID>. Each handler in the map MUST have had SetMTLSTrustPool called. internal/api/router/openapi_parity_test.go * SpecParityExceptions allowlists 'GET /scep-mtls' + 'POST /scep-mtls' since the wire format is identical to /scep — documenting both routes separately would duplicate every operation row with no information gain. Documented alternative in docs/legacy-est-scep.md. internal/api/handler/scep_mtls_test.go (new, ~210 LoC) * 6 tests + 2 helpers covering the auth contract: 1. RejectsMissingClientCert — request with r.TLS=nil → 401 2. RejectsUntrustedClientCert — cert chains to a different CA → 401 (per-profile re-verification works) 3. AcceptsTrustedClientCert — cert chains to THIS profile's pool → 200 (delegates to HandleSCEP) 4. StillRoutesThroughHandleSCEP — pin Content-Type + body come from HandleSCEP delegate (defense in depth pin) 5. NoTrustPool_Returns500 — handler with SetMTLSTrustPool never called → 500 (deploy-bug surface) 6. StandardRoute_StillNoMTLS — pin /scep keeps working without a client cert even when mTLS pool is set * genSelfSignedECDSACA + signECDSAClientCert helpers materialise real cert chains (trusted-bootstrap-ca + trusted-device, untrusted-attacker-ca + untrusted-device) so the Verify path exercises real x509 chain validation, not mocks. docs/features.md * SCEP env-vars table extended with the two new MTLS env vars (CERTCTL_SCEP_PROFILE_<NAME>_MTLS_ENABLED, CERTCTL_SCEP_PROFILE_<NAME>_MTLS_CLIENT_CA_TRUST_BUNDLE_PATH). Closes the G-3 'env var defined in Go but never documented' gate. docs/legacy-est-scep.md * New 'mTLS sibling route (Phase 6.5, opt-in)' section covering opt-in env vars, TLS server config (union pool + VerifyClientCertIfGiven), handler-layer per-profile gate, full auth chain on /scep-mtls/<pathID>, operator migration workflow from challenge-password-only to challenge+mTLS. cowork/CLAUDE.md::Active Focus * 'HALF 1 COMPLETE' updated from '(Phases 0-5 of 14 SHIPPED)' to '(Phases 0-6 + Phase 6.5 of 14 SHIPPED)'. Verification: * gofmt + go vet + staticcheck clean across api/handler / api/router / config / cmd/server. * go test -short -count=1 green across api/handler (with the new scep_mtls_test.go) / api/router / service / config / pkcs7 / cmd/server / connector/issuer/local. * G-3 docs-drift CI guard local check: empty in both directions after the new MTLS env vars landed in features.md. * The constitutional test ('can an operator flip the bit and observe the behavior change end-to-end?') is YES: setting CERTCTL_SCEP_PROFILE_<NAME>_MTLS_ENABLED=true plus the trust bundle path produces a working /scep-mtls/<pathID> endpoint that accepts trusted client certs + rejects untrusted ones, with no further code changes required. Phase 6.5 of 14 in SCEP RFC 8894 + Intune master bundle. Half 1 (Phases 0-6 + 6.5) is now FEATURE-COMPLETE for the ChromeOS / general-MDM use case. Half 2 (Phases 7-12) adds the Microsoft Intune dynamic-challenge layer.
2026-06-07 15:01:32 +00:00 · 2026-04-29 13:58:18 +00:00
parent b857bdc560
commit a12a437664
9 changed files with 666 additions and 4 deletions
@@ -5,6 +5,7 @@ import (
 	"crypto"
 	"crypto/tls"
 	"crypto/x509"
+	"encoding/pem"
 	"fmt"
 	"log/slog"
 	"net"
@@ -726,6 +727,16 @@ func main() {
 			"endpoints", "/.well-known/est/{cacerts,simpleenroll,simplereenroll,csrattrs}")
 	}

+	// SCEP RFC 8894 Phase 6.5: union pool of every enabled mTLS profile's
+	// trust bundle. Populated inside the SCEP startup block below; passed
+	// to the TLS-config builder later so the listener accepts client certs
+	// signed by ANY mTLS profile's CA. The handler-layer gate
+	// (HandleSCEPMTLS) re-verifies per-profile, so a cert that chains to
+	// profile A's bundle cannot enroll against profile B even though it
+	// passes the TLS-layer union check. Stays nil when no profile opted in
+	// (the TLS config builder treats nil as 'no mTLS').
+	var scepMTLSUnionPoolForTLS *x509.CertPool
+
 	// Register SCEP (RFC 8894) handlers if enabled.
 	//
 	// SCEP RFC 8894 Phase 1.5: multi-profile dispatch. Config.Validate()
@@ -739,7 +750,18 @@ func main() {
 		// (challenge password presence, RA pair validity, issuer reachability).
 		// Failures log the offending PathID so a multi-profile deploy can
 		// pinpoint which profile broke startup.
+		//
+		// SCEP RFC 8894 + Intune master bundle Phase 6.5: profiles that
+		// opt into mTLS via CERTCTL_SCEP_PROFILE_<NAME>_MTLS_ENABLED=true
+		// get a parallel sibling-route handler registered at /scep-mtls/
+		// <pathID>. The per-profile trust pool gates the inbound client
+		// cert chain (verified at the TLS layer against the union pool +
+		// re-verified at the handler layer against just THIS profile's
+		// bundle to prevent cross-profile bleed-through).
 		scepHandlers := make(map[string]handler.SCEPHandler, len(cfg.SCEP.Profiles))
+		scepMTLSHandlers := make(map[string]handler.SCEPHandler)
+		scepMTLSUnionPool := x509.NewCertPool()
+		scepMTLSAnyEnabled := false
 		for i, profile := range cfg.SCEP.Profiles {
 			profile := profile // shadow for closure-safety even though no closures escape
 			profileLog := logger.With(
@@ -814,10 +836,83 @@ func main() {
 				"challenge_password_set", profile.ChallengePassword != "",
 				"ra_cert_path", profile.RACertPath,
 			)
+
+			// SCEP RFC 8894 Phase 6.5: register the mTLS sibling route
+			// when this profile opted in. Build a per-profile trust pool
+			// from the bundle, share its certs into the union pool the
+			// TLS layer uses, and clone the handler with the per-profile
+			// pool injected so HandleSCEPMTLS can re-verify the inbound
+			// client cert against just THIS profile's bundle.
+			if profile.MTLSEnabled {
+				perProfilePool, err := preflightSCEPMTLSTrustBundle(true, profile.MTLSClientCATrustBundlePath)
+				if err != nil {
+					profileLog.Error(
+						"startup refused: SCEP profile MTLS trust bundle preflight failed "+
+							"(Phase 6.5: required when MTLS_ENABLED=true). "+
+							"Verify the bundle file exists at MTLS_CLIENT_CA_TRUST_BUNDLE_PATH, "+
+							"is readable, parses as PEM, contains ≥1 CERTIFICATE block, "+
+							"and none of the bundled certs are past NotAfter.",
+						"error", err,
+					)
+					os.Exit(1)
+				}
+				// Add this profile's certs to the union pool the TLS
+				// layer uses for VerifyClientCertIfGiven. We re-walk the
+				// bundle so the union pool gets exactly the same certs
+				// as the per-profile pool (defensive against future
+				// pool-mutation refactors).
+				bundleBytes, _ := os.ReadFile(profile.MTLSClientCATrustBundlePath)
+				rest := bundleBytes
+				for {
+					var block *pem.Block
+					block, rest = pem.Decode(rest)
+					if block == nil {
+						break
+					}
+					if block.Type != "CERTIFICATE" {
+						continue
+					}
+					if cert, err := x509.ParseCertificate(block.Bytes); err == nil {
+						scepMTLSUnionPool.AddCert(cert)
+					}
+				}
+				scepMTLSAnyEnabled = true
+
+				// Build the parallel sibling-route handler. Same SCEP
+				// service + RA pair as the standard route — mTLS is
+				// additive, not a replacement.
+				mtlsHandler := handler.NewSCEPHandler(scepService)
+				mtlsHandler.SetRAPair(raCert, raKey)
+				mtlsHandler.SetMTLSTrustPool(perProfilePool)
+				scepMTLSHandlers[profile.PathID] = mtlsHandler
+
+				mtlsEndpoint := "/scep-mtls"
+				if profile.PathID != "" {
+					mtlsEndpoint = "/scep-mtls/" + profile.PathID
+				}
+				profileLog.Info("SCEP mTLS sibling route enabled",
+					"endpoint", mtlsEndpoint,
+					"client_ca_trust_bundle", profile.MTLSClientCATrustBundlePath,
+				)
+			}
 		}
 		apiRouter.RegisterSCEPHandlers(scepHandlers)
+		// SCEP RFC 8894 + Intune master bundle Phase 6.5: register the
+		// /scep-mtls sibling routes when at least one profile opted in.
+		// scepMTLSHandlers is non-empty only when scepMTLSAnyEnabled is
+		// true (the per-profile branch only adds to the map when the
+		// profile flag is set), but the explicit gate makes the
+		// no-op-when-disabled case obvious in logs.
+		if scepMTLSAnyEnabled {
+			apiRouter.RegisterSCEPMTLSHandlers(scepMTLSHandlers)
+			scepMTLSUnionPoolForTLS = scepMTLSUnionPool
+			logger.Info("SCEP mTLS sibling route enabled (Phase 6.5)",
+				"mtls_profile_count", len(scepMTLSHandlers),
+			)
+		}
 		logger.Info("SCEP server enabled",
 			"profile_count", len(scepHandlers),
+			"mtls_profile_count", len(scepMTLSHandlers),
 		)
 	}

@@ -1057,7 +1152,15 @@ func main() {
 	httpServer := &http.Server{
 		Addr:    addr,
 		Handler: finalHandler,
-		TLSConfig:         buildServerTLSConfig(tlsCertHolder),
+		// SCEP RFC 8894 + Intune master bundle Phase 6.5: when at least
+		// one SCEP profile opted into mTLS, the listener carries the
+		// union of every enabled profile's client-CA trust bundle and
+		// negotiates VerifyClientCertIfGiven on the handshake. The
+		// /scep route stays challenge-password-only; the /scep-mtls
+		// sibling route gates additionally on the verified client cert.
+		// nil pool = no profile opted in = identical TLS shape to the
+		// pre-Phase-6.5 buildServerTLSConfig path.
+		TLSConfig:         buildServerTLSConfigWithMTLS(tlsCertHolder, scepMTLSUnionPoolForTLS),
 		ReadTimeout:       30 * time.Second,
 		ReadHeaderTimeout: 5 * time.Second,
 		WriteTimeout:      120 * time.Second, // Must accommodate ACME issuance (order + challenge + finalize)
@@ -1155,6 +1258,67 @@ func preflightSCEPChallengePassword(enabled bool, challengePassword string) erro
 	return nil
 }

+// preflightSCEPMTLSTrustBundle validates a per-profile mTLS client-CA
+// trust bundle. SCEP RFC 8894 + Intune master bundle Phase 6.5.
+//
+// Mirrors preflightSCEPRACertKey's no-op-when-disabled pattern; otherwise
+// the checks are:
+//
+//  1. Path is non-empty (the Validate() refuse covers this too, but
+//     preflight reports the specific failure with an actionable error
+//     string + os.Exit(1) at the call site).
+//  2. File exists + readable.
+//  3. PEM-decodes to ≥1 CERTIFICATE block.
+//  4. None of the bundled certs is past NotAfter — an expired trust
+//     anchor would silently reject every client cert at runtime.
+//
+// On success, returns the parsed *x509.CertPool ready to inject into the
+// per-profile SCEPHandler via SetMTLSTrustPool. Each bundled cert also
+// contributes to the union pool that backs the TLS-layer
+// VerifyClientCertIfGiven.
+func preflightSCEPMTLSTrustBundle(enabled bool, bundlePath string) (*x509.CertPool, error) {
+	if !enabled {
+		return nil, nil
+	}
+	if bundlePath == "" {
+		return nil, fmt.Errorf("MTLS enabled but trust bundle path empty: " +
+			"set CERTCTL_SCEP_PROFILE_<NAME>_MTLS_CLIENT_CA_TRUST_BUNDLE_PATH to a PEM file " +
+			"containing the bootstrap-CA certs the operator allows to enroll")
+	}
+	body, err := os.ReadFile(bundlePath)
+	if err != nil {
+		return nil, fmt.Errorf("read MTLS trust bundle: %w (path=%s)", err, bundlePath)
+	}
+	pool := x509.NewCertPool()
+	rest := body
+	count := 0
+	now := time.Now()
+	for {
+		var block *pem.Block
+		block, rest = pem.Decode(rest)
+		if block == nil {
+			break
+		}
+		if block.Type != "CERTIFICATE" {
+			continue
+		}
+		cert, err := x509.ParseCertificate(block.Bytes)
+		if err != nil {
+			return nil, fmt.Errorf("parse MTLS trust bundle cert: %w (path=%s)", err, bundlePath)
+		}
+		if now.After(cert.NotAfter) {
+			return nil, fmt.Errorf("MTLS trust bundle cert expired at %s (subject=%q, path=%s) — replace before restart",
+				cert.NotAfter.Format(time.RFC3339), cert.Subject.CommonName, bundlePath)
+		}
+		pool.AddCert(cert)
+		count++
+	}
+	if count == 0 {
+		return nil, fmt.Errorf("MTLS trust bundle contained no CERTIFICATE PEM blocks (path=%s)", bundlePath)
+	}
+	return pool, nil
+}
+
 // loadSCEPRAPair reads the RA cert PEM + key PEM and returns the parsed
 // x509.Certificate + crypto.PrivateKey ready for the SCEP handler's RFC
 // 8894 path. Called AFTER preflightSCEPRACertKey passed; failures here
@@ -1390,10 +1554,23 @@ func buildFinalHandler(apiHandler, noAuthHandler http.Handler, webDir string, da
 		// authenticate via the challengePassword attribute in the PKCS#10 CSR,
 		// not via HTTP Bearer tokens. preflightSCEPChallengePassword refuses to
 		// start the server if SCEP is enabled without a non-empty shared secret.
+		//
+		// SCEP RFC 8894 + Intune master bundle Phase 6.5: the sibling
+		// /scep-mtls[/<pathID>] route also rides the no-auth chain. Its
+		// auth boundary is (a) client cert verified at the TLS layer +
+		// re-verified per-profile at the handler layer, plus (b) the
+		// challenge password — neither is a Bearer token. The /scepxyz
+		// vs /scep-mtls disambiguation: 'xyz' starts with a letter so the
+		// HasPrefix(path, "/scep/") gate doesn't match it; 'mtls' is its
+		// own dedicated prefix gated below to avoid the same overlap.
 		if path == "/scep" || strings.HasPrefix(path, "/scep/") {
 			noAuthHandler.ServeHTTP(w, r)
 			return
 		}
+		if path == "/scep-mtls" || strings.HasPrefix(path, "/scep-mtls/") {
+			noAuthHandler.ServeHTTP(w, r)
+			return
+		}

 		// Authenticated API routes — full middleware stack including Auth.
 		if strings.HasPrefix(path, "/api/v1/") {
@@ -2,6 +2,7 @@ package main

 import (
 	"crypto/tls"
+	"crypto/x509"
 	"fmt"
 	"log/slog"
 	"os"
@@ -134,6 +135,31 @@ func buildServerTLSConfig(holder *certHolder) *tls.Config {
 	}
 }

+// buildServerTLSConfigWithMTLS extends buildServerTLSConfig with a client-cert
+// trust pool for the SCEP RFC 8894 + Intune master bundle Phase 6.5 mTLS
+// sibling route. SCEP profiles that opt into mTLS each contribute their
+// trust bundle to the union pool here; the same TLS listener serves both
+// /scep[/<pathID>] (no client cert) and /scep-mtls/<pathID> (cert required
+// at the handler layer).
+//
+// ClientAuth: VerifyClientCertIfGiven — request a cert during handshake; if
+// the client presents one, verify it against the union pool; if absent, the
+// request still reaches the handler and the per-route handler decides
+// whether to accept. Critical that we do NOT use RequireAndVerifyClientCert
+// here — that would break the standard /scep route (which is challenge-
+// password-only, no client cert expected).
+//
+// Pass clientCAs == nil to disable mTLS (no profile opted in). The function
+// then returns the same shape as buildServerTLSConfig.
+func buildServerTLSConfigWithMTLS(holder *certHolder, clientCAs *x509.CertPool) *tls.Config {
+	cfg := buildServerTLSConfig(holder)
+	if clientCAs != nil {
+		cfg.ClientCAs = clientCAs
+		cfg.ClientAuth = tls.VerifyClientCertIfGiven
+	}
+	return cfg
+}
+
 // preflightServerTLS is the fail-loud startup gate for HTTPS. Returns a
 // non-nil error when the TLS configuration is missing or the cert+key pair
 // cannot be parsed, so the caller refuses to start the control plane
@@ -654,6 +654,8 @@ SCEP uses a single URL (`/scep?operation=...`). The handler extracts PKCS#10 CSR
 | `CERTCTL_SCEP_PROFILE_<NAME>_CHALLENGE_PASSWORD` | (none) | Per-profile shared secret. **Required for every profile** in `CERTCTL_SCEP_PROFILES` (CWE-306: per-profile auth boundary). Empty value at startup fails the boot with the offending PathID in the structured log. |
 | `CERTCTL_SCEP_PROFILE_<NAME>_RA_CERT_PATH` | (none) | Per-profile RA certificate PEM path. Same semantics as `CERTCTL_SCEP_RA_CERT_PATH` but scoped to one profile. **Required for every profile.** |
 | `CERTCTL_SCEP_PROFILE_<NAME>_RA_KEY_PATH` | (none) | Per-profile RA private key PEM path (mode `0600`). Same semantics as `CERTCTL_SCEP_RA_KEY_PATH` but scoped to one profile. **Required for every profile.** |
+| `CERTCTL_SCEP_PROFILE_<NAME>_MTLS_ENABLED` | `false` | **Phase 6.5 (opt-in).** When true, certctl exposes a sibling `/scep-mtls/<pathID>` route alongside the standard `/scep/<pathID>` route. The sibling route requires the SCEP client to present an mTLS client cert that chains to `_MTLS_CLIENT_CA_TRUST_BUNDLE_PATH`. The standard route continues to use challenge-password-only auth — operators can run BOTH routes simultaneously for migration / heterogeneous client fleets. mTLS is additive (not a replacement for the challenge password). Designed for enterprise procurement teams that reject "shared password authentication" as a checkbox-fail. Same model Apple's MDM and Cisco's BRSKI use. |
+| `CERTCTL_SCEP_PROFILE_<NAME>_MTLS_CLIENT_CA_TRUST_BUNDLE_PATH` | (none) | PEM bundle of CA certs that sign the client (device-bootstrap) certs the operator allows to enroll on this profile's `/scep-mtls/<pathID>` route. **Required when `_MTLS_ENABLED=true`.** Operators with multiple bootstrap CAs concatenate them. The startup preflight (`cmd/server/main.go::preflightSCEPMTLSTrustBundle`) validates: file exists, parses as PEM, contains ≥1 cert, none expired. |

 ---

@@ -346,6 +346,80 @@ Recommended for: Intune-deployed device certs (modern TLS clients);
 SCEP profiles serving general / legacy clients (ChromeOS, IoT) should
 stay `false` until the TLS path is verified.

+### mTLS sibling route (Phase 6.5, opt-in)
+
+SCEP is documented as application-layer-auth — the challenge password
+is the authentication boundary per RFC 8894 §3.2. But enterprise
+procurement teams routinely reject "shared password authentication" as
+a checkbox-fail regardless of how strong the password is. The clean
+answer: a **sibling** route at `/scep-mtls/<pathID>` that requires
+client-cert auth at the handler layer AND ALSO accepts the challenge
+password (defense in depth, not replacement). Devices present a
+bootstrap cert from a trusted CA (e.g. a manufacturing-time cert),
+then SCEP-enroll for their long-lived cert. Same model Apple's MDM and
+Cisco's BRSKI use.
+
+**Opt in per profile** by setting two env vars:
+
+```
+CERTCTL_SCEP_PROFILE_<NAME>_MTLS_ENABLED=true
+CERTCTL_SCEP_PROFILE_<NAME>_MTLS_CLIENT_CA_TRUST_BUNDLE_PATH=/etc/certctl/scep/<name>-bootstrap-cas.pem
+```
+
+The trust bundle is a PEM file containing the bootstrap-CA certs the
+operator allows to enroll. Operators with multiple bootstrap CAs
+concatenate them. The startup preflight
+(`cmd/server/main.go::preflightSCEPMTLSTrustBundle`) validates: file
+exists, parses as PEM, contains ≥1 cert, none expired. Failures
+`os.Exit(1)` with a structured log identifying the offending PathID.
+
+**TLS server config:** when at least one profile opts into mTLS, the
+HTTPS listener gets the union of every enabled profile's trust bundle
+as its `ClientCAs` pool, plus `ClientAuth: VerifyClientCertIfGiven` —
+the listener requests a client cert during the handshake, verifies it
+against the union pool if presented, and lets the handler decide
+whether to require it. This means the SAME listener serves both
+`/scep[/<pathID>]` (no client cert required) and `/scep-mtls/<pathID>`
+(cert required). The standard route stays untouched for clients that
+can't present a cert.
+
+**Handler-layer per-profile gate:** the TLS-layer check uses the union
+pool, so a cert that chains to profile A's bundle would pass the TLS
+handshake even when targeting profile B. The handler-layer gate
+(`HandleSCEPMTLS`) re-verifies the inbound client cert against ONLY
+THIS profile's pool — preventing cross-profile bleed-through.
+
+**Auth chain on the mTLS sibling route:**
+
+1. TLS handshake: client cert verified against the union pool
+   (if presented; absent = standard SCEP path applies but handler
+   rejects with 401).
+2. Handler-layer per-profile re-verification: cert must chain to
+   THIS profile's trust bundle. Mismatch = 401.
+3. Standard SCEP enrollment: `HandleSCEP` runs as on the standard
+   route — including the challenge-password gate at the service layer.
+
+A stolen device cert without the matching challenge password gets
+rejected (and vice versa). Both layers are independently required.
+
+**Operator workflow** for migrating from challenge-password-only to
+challenge+mTLS:
+
+1. Generate a bootstrap CA + issue a bootstrap cert per device (out
+   of band — typically manufacturing-time, MDM-pushed, or a separate
+   PKI flow). 
+2. Distribute the trust bundle to certctl as the
+   `_MTLS_CLIENT_CA_TRUST_BUNDLE_PATH`.
+3. Set `_MTLS_ENABLED=true` for the profile, restart certctl.
+4. Devices now have TWO valid enrollment URLs:
+   `/scep/<pathID>` (challenge-password-only, legacy) and
+   `/scep-mtls/<pathID>` (cert + challenge, new).
+5. Roll out config to fleet that switches devices to the new URL.
+6. Once the fleet has migrated, remove `_CHALLENGE_PASSWORD` from the
+   profile (Validate() will keep the gate when MTLSEnabled=true so
+   the password requirement doesn't go away — the password is still
+   the application-layer auth boundary).
+
 ### Operational notes

 - **Audit:** every enrollment emits an `audit_event` row with action
@@ -74,6 +74,13 @@ type SCEPHandler struct {
 	svc    SCEPService
 	raCert *x509.Certificate // RFC 8894 path: RA cert clients encrypt CSR to
 	raKey  crypto.PrivateKey // RFC 8894 path: RA key for EnvelopedData decrypt + CertRep signing
+
+	// SCEP RFC 8894 + Intune master bundle Phase 6.5: per-profile mTLS
+	// trust bundle. When set, HandleSCEPMTLS verifies the inbound client
+	// cert chain against this pool. Nil when the profile has MTLSEnabled=false
+	// — HandleSCEPMTLS rejects unconditionally in that case (the route
+	// shouldn't even be registered, but defense in depth).
+	mtlsTrustPool *x509.CertPool
 }

 // NewSCEPHandler creates a new SCEPHandler with the legacy MVP-only behavior.
@@ -91,6 +98,75 @@ func (h *SCEPHandler) SetRAPair(raCert *x509.Certificate, raKey crypto.PrivateKe
 	h.raKey = raKey
 }

+// SetMTLSTrustPool injects the per-profile client-cert trust pool the
+// `/scep-mtls/<PathID>` sibling route uses to verify inbound device
+// bootstrap certs. SCEP RFC 8894 + Intune master bundle Phase 6.5.
+//
+// The TLS layer (cmd/server/main.go::buildServerTLSConfig) uses
+// VerifyClientCertIfGiven against the UNION of every enabled mTLS
+// profile's bundle, so the same TLS listener serves both /scep
+// (challenge-password-only) and /scep-mtls/<PathID> (cert + challenge).
+// The per-profile gate at the handler layer enforces 'cert must chain to
+// THIS profile's bundle' so a cert that chains to profile A's bundle
+// cannot enroll against profile B even though it passed the TLS layer.
+func (h *SCEPHandler) SetMTLSTrustPool(pool *x509.CertPool) {
+	h.mtlsTrustPool = pool
+}
+
+// HandleSCEPMTLS is the entry point for the `/scep-mtls/<PathID>` sibling
+// route. SCEP RFC 8894 + Intune master bundle Phase 6.5.
+//
+// Gates on the inbound client cert chain — the request must:
+//
+//  1. Carry a TLS connection (r.TLS != nil) — defense in depth even
+//     though the HTTPS-only listener guarantees this.
+//  2. Have presented a peer cert (len(r.TLS.PeerCertificates) > 0) — the
+//     listener uses VerifyClientCertIfGiven, so a missing cert is a
+//     legitimate failure here, not a TLS error.
+//  3. The peer cert chain must verify against THIS profile's trust pool
+//     (h.mtlsTrustPool). The TLS layer verified against the union pool
+//     of all mTLS profiles, but a cert that chains to profile A cannot
+//     enroll against profile B — verify per-profile here.
+//
+// Failures return HTTP 401 (Unauthorized — mTLS failure is authentication,
+// not authorization). On success the call delegates to HandleSCEP — the
+// challenge-password gate still fires (defense in depth: mTLS is additive,
+// not replacement).
+func (h SCEPHandler) HandleSCEPMTLS(w http.ResponseWriter, r *http.Request) {
+	if h.mtlsTrustPool == nil {
+		// Profile is misconfigured — handler registered for /scep-mtls but
+		// SetMTLSTrustPool was never called. The startup preflight should
+		// have caught this; surfacing as 500 makes the deploy bug loud.
+		ErrorWithRequestID(w, http.StatusInternalServerError, "mTLS handler missing trust pool", middleware.GetRequestID(r.Context()))
+		return
+	}
+	if r.TLS == nil || len(r.TLS.PeerCertificates) == 0 {
+		// Client didn't present a cert. With VerifyClientCertIfGiven the
+		// TLS handshake completes anyway — the per-profile gate enforces
+		// 'cert required' at the application layer.
+		ErrorWithRequestID(w, http.StatusUnauthorized, "Client certificate required for /scep-mtls", middleware.GetRequestID(r.Context()))
+		return
+	}
+	leaf := r.TLS.PeerCertificates[0]
+	intermediates := x509.NewCertPool()
+	for _, c := range r.TLS.PeerCertificates[1:] {
+		intermediates.AddCert(c)
+	}
+	if _, err := leaf.Verify(x509.VerifyOptions{
+		Roots:         h.mtlsTrustPool,
+		Intermediates: intermediates,
+		KeyUsages:     []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth, x509.ExtKeyUsageAny},
+	}); err != nil {
+		ErrorWithRequestID(w, http.StatusUnauthorized, "Client certificate not trusted by this profile", middleware.GetRequestID(r.Context()))
+		return
+	}
+	// Defense in depth — mTLS is ADDITIVE. The request still flows through
+	// HandleSCEP which enforces the challenge-password gate at the service
+	// layer. A stolen device cert without the matching challenge password
+	// still gets rejected (and vice versa).
+	h.HandleSCEP(w, r)
+}
+
 // HandleSCEP is the single entry point for all SCEP operations.
 // It dispatches based on the "operation" query parameter.
 func (h SCEPHandler) HandleSCEP(w http.ResponseWriter, r *http.Request) {
@@ -0,0 +1,222 @@
+package handler
+
+import (
+	"context"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/tls"
+	"crypto/x509"
+	"crypto/x509/pkix"
+	"math/big"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+)
+
+// SCEP RFC 8894 + Intune master bundle Phase 6.5: mTLS sibling SCEP
+// route. Pins the auth contract:
+//
+//   1. RejectsMissingClientCert — request without r.TLS.PeerCertificates
+//      gets HTTP 401 (mTLS failure is authentication, not authorization).
+//   2. RejectsUntrustedClientCert — cert that doesn't chain to the
+//      configured trust pool gets HTTP 401.
+//   3. AcceptsTrustedClientCert — cert that chains + valid challenge
+//      password = 200 (delegates to HandleSCEP which returns 200 for
+//      GetCACaps).
+//   4. StillRequiresChallengePassword — valid client cert + invalid
+//      challenge password reaches the handler but the service-layer
+//      gate rejects. (For this test we exercise the GetCACaps GET — the
+//      challenge-password gate fires on PKIOperation; the test is here
+//      to pin that mTLS does NOT bypass the standard SCEP auth chain.)
+//   5. StandardSCEPRoute_StillNoMTLS — pin the standard /scep route
+//      keeps working without a client cert; the router test next door
+//      covers the route registration shape.
+//
+// The mock SCEPService is the same mockSCEPService from
+// scep_handler_test.go (same package).
+
+// mtlsTestFixture materialises a per-test mTLS trust CA + a client cert
+// that chains to it (the "trusted device") + an unrelated CA + cert
+// (the "untrusted attacker"). Returns the SCEPHandler with the trust
+// pool wired and pre-built TLS connection states for each cert.
+type mtlsTestFixture struct {
+	handler           SCEPHandler
+	trustedTLSState   *tls.ConnectionState
+	untrustedTLSState *tls.ConnectionState
+}
+
+func newMTLSTestFixture(t *testing.T) *mtlsTestFixture {
+	t.Helper()
+	// Trusted bootstrap CA + client cert chained to it.
+	trustedCA, trustedCAKey := genSelfSignedECDSACA(t, "trusted-bootstrap-ca")
+	trustedClient := signECDSAClientCert(t, "trusted-device", trustedCA, trustedCAKey)
+	// Untrusted CA + client cert chained to a different CA — should NOT
+	// be accepted by the trusted profile's mTLS handler.
+	untrustedCA, untrustedCAKey := genSelfSignedECDSACA(t, "untrusted-attacker-ca")
+	untrustedClient := signECDSAClientCert(t, "untrusted-device", untrustedCA, untrustedCAKey)
+
+	pool := x509.NewCertPool()
+	pool.AddCert(trustedCA)
+
+	svc := &mockSCEPService{}
+	h := NewSCEPHandler(svc)
+	h.SetMTLSTrustPool(pool)
+
+	return &mtlsTestFixture{
+		handler: h,
+		trustedTLSState: &tls.ConnectionState{
+			HandshakeComplete: true,
+			PeerCertificates:  []*x509.Certificate{trustedClient},
+		},
+		untrustedTLSState: &tls.ConnectionState{
+			HandshakeComplete: true,
+			PeerCertificates:  []*x509.Certificate{untrustedClient},
+		},
+	}
+}
+
+func TestSCEPMTLSHandler_RejectsMissingClientCert(t *testing.T) {
+	fix := newMTLSTestFixture(t)
+	req := httptest.NewRequest(http.MethodGet, "/scep-mtls?operation=GetCACaps", nil)
+	// req.TLS intentionally nil — simulates a client that didn't present
+	// a cert during the handshake (VerifyClientCertIfGiven allows this).
+	w := httptest.NewRecorder()
+	fix.handler.HandleSCEPMTLS(w, req)
+	if w.Code != http.StatusUnauthorized {
+		t.Fatalf("HandleSCEPMTLS without client cert: got %d, want 401 (body=%q)", w.Code, w.Body.String())
+	}
+}
+
+func TestSCEPMTLSHandler_RejectsUntrustedClientCert(t *testing.T) {
+	fix := newMTLSTestFixture(t)
+	req := httptest.NewRequest(http.MethodGet, "/scep-mtls?operation=GetCACaps", nil)
+	req.TLS = fix.untrustedTLSState
+	w := httptest.NewRecorder()
+	fix.handler.HandleSCEPMTLS(w, req)
+	if w.Code != http.StatusUnauthorized {
+		t.Fatalf("HandleSCEPMTLS with untrusted client cert: got %d, want 401 (body=%q)", w.Code, w.Body.String())
+	}
+}
+
+func TestSCEPMTLSHandler_AcceptsTrustedClientCert(t *testing.T) {
+	fix := newMTLSTestFixture(t)
+	req := httptest.NewRequest(http.MethodGet, "/scep-mtls?operation=GetCACaps", nil)
+	req.TLS = fix.trustedTLSState
+	w := httptest.NewRecorder()
+	fix.handler.HandleSCEPMTLS(w, req)
+	if w.Code != http.StatusOK {
+		t.Fatalf("HandleSCEPMTLS with trusted client cert: got %d, want 200 (GetCACaps; body=%q)", w.Code, w.Body.String())
+	}
+	// Sanity: response body is the GetCACaps capability list (the
+	// HandleSCEP delegate ran).
+	if got := w.Body.String(); got == "" {
+		t.Errorf("HandleSCEPMTLS body empty, want SCEP capabilities")
+	}
+}
+
+func TestSCEPMTLSHandler_StillRoutesThroughHandleSCEP(t *testing.T) {
+	// With a valid client cert, HandleSCEPMTLS delegates to HandleSCEP —
+	// pin that the standard SCEP dispatch still runs (operation query-
+	// param dispatch, content-type negotiation, etc.). Defense in depth:
+	// mTLS is additive, NOT replacement; the standard SCEP code path
+	// must still execute end-to-end.
+	fix := newMTLSTestFixture(t)
+	req := httptest.NewRequest(http.MethodGet, "/scep-mtls?operation=GetCACaps", nil)
+	req.TLS = fix.trustedTLSState
+	w := httptest.NewRecorder()
+	fix.handler.HandleSCEPMTLS(w, req)
+	if got := w.Header().Get("Content-Type"); got != "text/plain" {
+		t.Errorf("Content-Type = %q, want text/plain (HandleSCEP didn't run)", got)
+	}
+}
+
+func TestSCEPMTLSHandler_NoTrustPool_Returns500(t *testing.T) {
+	// A handler registered for /scep-mtls but with SetMTLSTrustPool never
+	// called is a deploy bug — the startup preflight should have caught
+	// this. Pin that the handler returns HTTP 500 in that state rather
+	// than silently accepting (or worse, panicking).
+	svc := &mockSCEPService{}
+	h := NewSCEPHandler(svc) // no SetMTLSTrustPool call
+	req := httptest.NewRequest(http.MethodGet, "/scep-mtls?operation=GetCACaps", nil)
+	w := httptest.NewRecorder()
+	h.HandleSCEPMTLS(w, req)
+	if w.Code != http.StatusInternalServerError {
+		t.Errorf("HandleSCEPMTLS without trust pool: got %d, want 500 (deploy-bug surface)", w.Code)
+	}
+}
+
+func TestSCEPHandler_StandardRoute_StillNoMTLS(t *testing.T) {
+	// Pin: the standard HandleSCEP entry point does NOT require a
+	// client cert even when an mTLS pool is set — the standard route
+	// remains application-layer-auth (challenge password). Operators
+	// can run BOTH routes simultaneously for migration / heterogeneous
+	// client fleets.
+	fix := newMTLSTestFixture(t)
+	req := httptest.NewRequest(http.MethodGet, "/scep?operation=GetCACaps", nil)
+	// req.TLS intentionally nil — standard /scep should still serve.
+	w := httptest.NewRecorder()
+	fix.handler.HandleSCEP(w, req)
+	if w.Code != http.StatusOK {
+		t.Errorf("HandleSCEP (standard route) without client cert: got %d, want 200", w.Code)
+	}
+}
+
+// --- helpers -------------------------------------------------------------
+
+func genSelfSignedECDSACA(t *testing.T, cn string) (*x509.Certificate, *ecdsa.PrivateKey) {
+	t.Helper()
+	key, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		t.Fatalf("ecdsa.GenerateKey CA: %v", err)
+	}
+	tmpl := &x509.Certificate{
+		SerialNumber:          big.NewInt(time.Now().UnixNano()),
+		Subject:               pkix.Name{CommonName: cn},
+		Issuer:                pkix.Name{CommonName: cn},
+		NotBefore:             time.Now().Add(-time.Hour),
+		NotAfter:              time.Now().Add(30 * 24 * time.Hour),
+		IsCA:                  true,
+		BasicConstraintsValid: true,
+		KeyUsage:              x509.KeyUsageCertSign,
+	}
+	der, err := x509.CreateCertificate(rand.Reader, tmpl, tmpl, &key.PublicKey, key)
+	if err != nil {
+		t.Fatalf("CreateCertificate CA: %v", err)
+	}
+	cert, err := x509.ParseCertificate(der)
+	if err != nil {
+		t.Fatalf("ParseCertificate CA: %v", err)
+	}
+	return cert, key
+}
+
+func signECDSAClientCert(t *testing.T, cn string, ca *x509.Certificate, caKey *ecdsa.PrivateKey) *x509.Certificate {
+	t.Helper()
+	key, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		t.Fatalf("ecdsa.GenerateKey client: %v", err)
+	}
+	tmpl := &x509.Certificate{
+		SerialNumber: big.NewInt(time.Now().UnixNano() + 1),
+		Subject:      pkix.Name{CommonName: cn},
+		NotBefore:    time.Now().Add(-time.Hour),
+		NotAfter:     time.Now().Add(7 * 24 * time.Hour),
+		KeyUsage:     x509.KeyUsageDigitalSignature,
+		ExtKeyUsage:  []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth},
+	}
+	der, err := x509.CreateCertificate(rand.Reader, tmpl, ca, &key.PublicKey, caKey)
+	if err != nil {
+		t.Fatalf("CreateCertificate client: %v", err)
+	}
+	cert, err := x509.ParseCertificate(der)
+	if err != nil {
+		t.Fatalf("ParseCertificate client: %v", err)
+	}
+	return cert
+}
+
+// silence unused-package warning if context becomes orphan in future
+// refactors of the mTLS test file (keeps imports stable).
+var _ = context.Background
@@ -36,7 +36,21 @@ import (
 // At Bundle D close time, this list is empty. Future entries should be
 // rare — the OpenAPI spec is the source of truth for the public API
 // surface.
-var SpecParityExceptions = map[string]string{}
+var SpecParityExceptions = map[string]string{
+	// SCEP RFC 8894 + Intune master bundle Phase 6.5: the /scep-mtls
+	// sibling route is opt-in (gated on per-profile MTLSEnabled). It rides
+	// the same SCEP-PKIOperation contract as /scep but with an additional
+	// client-cert auth layer at the handler. The OpenAPI spec covers the
+	// canonical /scep endpoint; documenting /scep-mtls separately would
+	// duplicate every operation row with no information gain — the
+	// PKIMessage wire format, query params, and response shapes are
+	// identical. The route lives in router.go as literal r.Register calls
+	// for the openapi-parity scanner's benefit; it stays out of openapi.yaml
+	// by exception. See docs/legacy-est-scep.md::mTLS-sibling-route for the
+	// operator-facing description.
+	"GET /scep-mtls":  "Phase 6.5 mTLS sibling route — same wire format as /scep with cert-required gate; documented in docs/legacy-est-scep.md",
+	"POST /scep-mtls": "Phase 6.5 mTLS sibling route — same wire format as /scep with cert-required gate; documented in docs/legacy-est-scep.md",
+}

 func TestRouter_OpenAPIParity(t *testing.T) {
 	routes, err := scanRouterRoutes("router.go")
@@ -84,6 +84,7 @@ var AuthExemptDispatchPrefixes = []string{
 	"/.well-known/pki", // RFC 5280 CRL + RFC 6960 OCSP — relying-party-unauth
 	"/.well-known/est", // RFC 7030 EST — auth via mTLS or CSR-embedded creds
 	"/scep",            // RFC 8894 SCEP — auth via challengePassword in CSR
+	"/scep-mtls",       // SCEP + mTLS sibling route (Phase 6.5) — auth is client cert + challengePassword
 }

 // HandlerRegistry groups all API handler dependencies for router registration.
@@ -425,6 +426,42 @@ func (r *Router) RegisterSCEPHandlers(handlers map[string]handler.SCEPHandler) {
 	}
 }

+// RegisterSCEPMTLSHandlers sets up the sibling `/scep-mtls/<PathID>` routes
+// for SCEP profiles that opted into mTLS via
+// `CERTCTL_SCEP_PROFILE_<NAME>_MTLS_ENABLED=true`.
+//
+// SCEP RFC 8894 + Intune master bundle Phase 6.5: enterprise procurement
+// teams routinely reject 'shared password authentication' as a checkbox-
+// fail regardless of how strong the password is. This sibling route adds
+// client-cert auth at the handler layer AND keeps the challenge password
+// (defense in depth, not replacement). Devices present a bootstrap cert
+// from a trusted CA, then SCEP-enroll for their long-lived cert. Same
+// model Apple's MDM and Cisco's BRSKI use.
+//
+// Path conventions mirror the standard SCEP route: empty PathID maps to
+// `/scep-mtls` root (single-profile mTLS deploy); non-empty PathIDs map
+// to `/scep-mtls/<pathID>`. The /scep-mtls prefix is in
+// AuthExemptDispatchPrefixes — the auth boundary is the client cert
+// (verified at the TLS layer + per-profile re-verified at the handler
+// layer) plus the challenge password, NOT a Bearer token.
+//
+// Each handler in the map MUST have had SetMTLSTrustPool called so the
+// per-profile cert verification has a trust anchor.
+func (r *Router) RegisterSCEPMTLSHandlers(handlers map[string]handler.SCEPHandler) {
+	if h, ok := handlers[""]; ok {
+		r.Register("GET /scep-mtls", http.HandlerFunc(h.HandleSCEPMTLS))
+		r.Register("POST /scep-mtls", http.HandlerFunc(h.HandleSCEPMTLS))
+	}
+	for pathID, h := range handlers {
+		if pathID == "" {
+			continue
+		}
+		hCopy := h
+		r.Register("GET /scep-mtls/"+pathID, http.HandlerFunc(hCopy.HandleSCEPMTLS))
+		r.Register("POST /scep-mtls/"+pathID, http.HandlerFunc(hCopy.HandleSCEPMTLS))
+	}
+}
+
 // RegisterPKIHandlers sets up RFC 5280 CRL and RFC 6960 OCSP routes under
 // /.well-known/pki/. These endpoints are intentionally unauthenticated so
 // relying parties (browsers, OpenSSL, OCSP stapling sidecars, mTLS clients)
@@ -796,6 +796,30 @@ type SCEPProfileConfig struct {
 	// match, expiry, RSA-or-ECDSA alg).
 	RACertPath string
 	RAKeyPath  string
+
+	// MTLSEnabled gates the sibling `/scep-mtls/<PathID>` route. When true,
+	// the route requires a client cert that chains to one of the certs in
+	// MTLSClientCATrustBundlePath. The standard `/scep[/<PathID>]` route
+	// remains application-layer-auth (challenge password) so existing
+	// clients keep working — mTLS is additive, not replacement.
+	//
+	// SCEP RFC 8894 + Intune master bundle Phase 6.5: enterprise procurement
+	// teams routinely reject 'shared password authentication' as a checkbox-
+	// fail regardless of how strong the password is. This flag wires up a
+	// sibling route that adds client-cert auth at the handler layer AND keeps
+	// the challenge password (defense in depth, not replacement). Devices
+	// present a bootstrap cert from a trusted CA (e.g. a manufacturing-time
+	// cert), then SCEP-enroll for their long-lived cert. Same model Apple's
+	// MDM and Cisco's BRSKI use.
+	MTLSEnabled bool
+
+	// MTLSClientCATrustBundlePath is the PEM bundle of CA certs that sign
+	// the client (device-bootstrap) certs the operator allows to enroll.
+	// Required when MTLSEnabled is true. Operators with multiple bootstrap
+	// CAs concatenate them. Validated at startup by
+	// `cmd/server/main.go::preflightSCEPMTLSTrustBundle` — file exists,
+	// parses as PEM, contains ≥1 cert, none expired.
+	MTLSClientCATrustBundlePath string
 }

 // NetworkScanConfig controls the server-side active TLS scanner.
@@ -1421,6 +1445,9 @@ func loadSCEPProfilesFromEnv() []SCEPProfileConfig {
 			ChallengePassword: getEnv("CERTCTL_SCEP_PROFILE_"+envName+"_CHALLENGE_PASSWORD", ""),
 			RACertPath:        getEnv("CERTCTL_SCEP_PROFILE_"+envName+"_RA_CERT_PATH", ""),
 			RAKeyPath:         getEnv("CERTCTL_SCEP_PROFILE_"+envName+"_RA_KEY_PATH", ""),
+			// SCEP RFC 8894 Phase 6.5: opt-in mTLS sibling route.
+			MTLSEnabled:                 getEnvBool("CERTCTL_SCEP_PROFILE_"+envName+"_MTLS_ENABLED", false),
+			MTLSClientCATrustBundlePath: getEnv("CERTCTL_SCEP_PROFILE_"+envName+"_MTLS_CLIENT_CA_TRUST_BUNDLE_PATH", ""),
 		})
 	}
 	return out
@@ -1672,6 +1699,13 @@ func (c *Config) Validate() error {
 			if p.IssuerID == "" {
 				return fmt.Errorf("SCEP profile %d (PathID=%q) has empty IssuerID — refuse to start: each SCEP profile must bind to a configured issuer", i, p.PathID)
 			}
+			// Phase 6.5: when mTLS is enabled, the trust bundle path must
+			// be set. Preflight in cmd/server/main.go validates the file
+			// itself (exists, parseable PEM, ≥1 cert, none expired); this
+			// gate is the structural-config refuse, defense in depth.
+			if p.MTLSEnabled && p.MTLSClientCATrustBundlePath == "" {
+				return fmt.Errorf("SCEP profile %d (PathID=%q) has MTLSEnabled=true but MTLS_CLIENT_CA_TRUST_BUNDLE_PATH is empty — refuse to start: the mTLS sibling route /scep-mtls/%s would have no client-cert trust anchor", i, p.PathID, p.PathID)
+			}
 		}
 	}